Nano-molding process

ABSTRACT

A nano-molding process including an imprint process that replicates features sizes less than 7 nanometers. The nano-molding process produces a line edge roughness of the replicated features that is less than 2 nanometers. The nano-molding process including the steps of: a) forming a first substrate having nano-scale features formed thereon, b) casting at least one polymer against the substrate, c) curing the at least one polymer forming a mold, d) removing the mold from the first substrate, e) providing a second substrate having a molding material applied thereon, f) pressing the mold against the second substrate allowing the molding material to conform to a shape of the mold, g) curing the molding material, and h) removing the mold from the second substrate having the cured molding material revealing a replica of the first substrate.

FIELD OF THE INVENTION

The invention relates to a process for nano-molding.

BACKGROUND OF THE INVENTION

New techniques for fabricating structures with nanometer dimensions arecritically important to advances in nanoscience and technology. As thedemand for smaller electronic devices, and biological analysisapparatus, has increased, a need has been created for improvedfabrication processes for making such devices. The processes may beutilized in the fabrication of electronic, magnetic, mechanical, andoptical devices, as well as devices for biological and chemicalanalysis. The processes may be used, for example, to define the featuresand configurations of microcircuits, as well as, the structure andoperating features of optical waveguides and components.

These processes may also play a crucial role in the semiconductorindustry, replacing conventional projection mode photolithography, whosepractical limits make it impossible to reach resolution at sizes lessthan 45 nanometers. Projection mode photolithography is a method ofpatterning features, wherein a thin layer of photoresist is applied to asubstrate surface and selected portions of the resist are exposed to apattern of light. The resist is then developed to reveal a desiredpattern of exposed substrate for further processing, such as etching. Adifficulty with this process is that resolution is limited by thewavelength of the light, scattering in the resist and substrate, and thethickness and properties of the resist. As a result, projection modephotolithography cannot be utilized to economically create feature sizesless than 100 nanometers.

Next generation lithography (NGL) methods including e-beam, dip pen andnano-imprint techniques are being explored. E-beam methods includecreating patterns in polymers, called resists, and usingmicrolithography based on short wavelength UV radiation or electronbeams. Patterns are formed due to a change in solubility of polymersfrom exposure to the imaging radiation with the use of a solvent toremove a portion of the polymer film. However, large scale commerciallyproducing dimensions on a length-scale of less than 100 nm using thesetechniques is costly and can be carried out using very special imagingtools and materials.

Of the NGL techniques, those that use molds to imprint features intothin polymer films have attracted considerable attention. Although thewell defined optics associated with photolithographic techniques allowstheir resolution to be specified accurately, the resolution limits ofNGLs based on nano-molding are much more difficult to determine. Theuncertain polymer physics that governs the molding process and theabsence of a reliable means to evaluate the resolution at less than 5nanometers in length represent some limits on current techniques.

There is therefore, a need in the art for a nano-molding process thatsolves the problems outlined above and can be utilized to produce reliefstructures that have lateral and vertical dimensions less than 10nanometers. There is also a need in the art for such a process thatallows for verification of dimensions on a part produced by the process.

SUMMARY OF THE INVENTION

A nano-molding process including an imprint process that replicatesfeatures sizes less than 7 nanometers. The nano-molding process producesa line edge roughness of the replicated features that is less than 2nanometers. The nano-molding process including the steps of: a) forminga first substrate having nano-scale features formed thereon, b) castingat least one polymer against the substrate, c) curing the at least onepolymer forming a mold, d) removing the mold from the first substrate,e) providing a second substrate having a molding material appliedthereon, f) pressing the mold against the second substrate allowing themolding material to conform to a shape of the mold, g) curing themolding material, and h) removing the mold from the second substratehaving the cured molding material revealing a replica of the firstsubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (a-c) is a graphical representation of the process ofnano-molding of the present invention.

FIG. 2 (a-d) is an atomic force microscopy image of a single walledcarbon tube of the first substrate (a) and three separate replicasproduced by the process of the present invention (b-d);

FIG. 3 (a-c) includes an atomic force microscopy image of a replicaformed by the process of the present invention (a) and two transmissionelectron microscope images, one of a replica formed by the process ofthe present invention (b), and a single walled carbon tube of the firstsubstrate (c);

FIG. 4( a-h) includes a graph of the heights of features formed in themolding material that are associated with individual carbon nanotubeswith diameters of 2, 1.3 and 0.9 nanometers (a), a graph of the lengthaveraged height of features on the replica as a function of the singlewalled carbon nanotube diameter (b), atomic force microscopy images ofindividual carbon nanotubes with diameters of 2, 1.3 and 0.9 (c-e), andatomic force microscopy images of replicas associated with individualcarbon nanotubes with diameters of 2, 1.3 and 0.9 (f-h).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown a graphical representation of thenano-molding process of the present invention. The process includes thesteps of: a) forming a first substrate having nano-scale features formedthereon, b) casting at least one polymer against the first substrate, c)curing the at least one polymer forming a mold, d) removing the moldfrom the first substrate, e) providing a second substrate having amolding material applied thereon, f) pressing the mold against thesecond substrate allowing the molding material to conform to a shape ofthe mold, g) curing the molding material, and h) removing the mold fromthe second substrate having the cured molding material revealing areplica of the first substrate.

In a preferred aspect of the invention, the first substrate is a siliconwafer having single walled carbon nanotubes (SWNT) formed thereon. Thesilicon wafer may include a layer of silicon dioxide (SiO₂) formedthereon to promote adhesion and formation of the SWNTs. The firstsubstrate may also include other materials, such as, molds made withe-beam lithography, X-ray lithography, or a biological material on aplastic sheet. The first substrate preferably includes high qualitysub-monolayers of small diameter SWNTs that serve as templates fromwhich nanomolds can be constructed. The cylindrical cross sections andhigh aspect ratios of the tubes, the atomic scale uniformity of theirdimensions over lengths of many microns, their chemical inertness andthe ability to grow or deposit them in large quantities over large areason a range of substrates makes the SWNTs suitable for use in the processof the present invention.

The SWNTs may be formed using methane based chemical vapor depositionusing a relatively high concentration of ferritin catalysts. The SWNTsformed may have diameters of from 0.5 to 10 nm and preferably havediameters between 0.5 and 5 nm and a coverage of 1-10 tubes/m² onSiO₂/Si wafers. The continuous range of diameters of the tubes and theirrelatively high, but sub-monolayer, coverage make them ideal forevaluating resolution or dimension limits. The cylindrical geometry ofthe SWNTs allows their dimensions to be characterized simply by atomicforce microscope (AFM) measurements of their heights. The SWNTs arebound to the SiO₂/Si wafers by van der Waals adhesion forces that bindthe SWNTs to the substrate with sufficient strength to prevent theirremoval when a cured polymer mold is peeled away, as will be discussedin more detail below. Preferably the SWNTs have an absence of polymericresidue on large regions allowing for the replication of fine resolutionfeatures formed on the first substrate. The lack of polymeric residueindicates that the mold did not contaminate the master, thus thefeatures in the mold are due to true replication and not materialfailure. Optionally, the SWNT formed on the first substrate may includea layer of a silane applied thereon, to act as a release agent,preventing adhesion of a polymer used to form a mold of the firstsubstrate.

Following forming the first substrate, at least one polymer is cast andcured against the first substrate to form a mold. In a preferred aspectof the present invention, the mold is a composite mold having multiplepolymer layers. The first layer applied against the first substrate is arelatively high modulus (˜10 MPa) elastomer based onpolydimethylsiloxane, (h-PDMS). The h-PDMS is preferably prepared bymixing vinylmethylsiloxane-dimethylsiloxane,1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane, and a platinumcatalyst. Next, methylhydrosiloxane-dimethylsiloxane is added and mixedforming a prepolymer mixture of h-PDMS. The prepolymer mixture of h-PDMSmay be cast by spin casting or otherwise depositing the prepolymermixture of h-PDMS on the first substrate. The prepolymer mixture ofh-PDMS is partially cured by the platinum catalyst that induces additionof SiH bonds across vinyl groups in the prepolymer mixture of h-PDMS,forming SiCH₂—CH₂—Si linkages (also known as hydrosilylation). Themultiple reaction sites on both the base and crosslinking oligomers inthe prepolymer mixture of h-PDMS allows for 3D crosslinking prohibitingrelative movement among bonded atoms. The low viscosity (˜1000 cP atroom temperature) of the prepolymer mixture of h-PDMS and theconformability of the silicone backbone allows for replication of finefeatures.

A second polymer is then applied to a back of the partially cured h-PDMSlayer. Preferably, a physically tough, low modulus PDMS (s-PDMS) layeris applied to the partially cured h-PDMS layer to make the mold easy tohandle. In a preferred aspect of the present invention the s-PDMS isSylgard 184 commercially available from Dow Corning Corporation. Afterapplication of the s-PDMS layer, the multiple layers of polymer on thefirst substrate may be fully cured to form a composite mold.

Following formation of the mold, a molding material is applied to asecond substrate to which the mold is pressed, allowing the moldingmaterial to conform to the shape of the mold. The second substratepreferably is a SiO₂/Si wafer, as previously described in relation tothe first substrate. Other materials may also be utilized includingwafers, plastic films, glass plates or other materials suitable for thepurpose of acting as a substrate.

The molding material can be a prepolymer, monomer or any polymer capableof molding with the composite mold, and posses the necessarycharacteristics for analysis. Preferably the molding material is apolyurethane prepolymer (PU) and even more preferably a photo-curable orUltraviolet curable PU polymer. Additionally, the molding material maybe polyacrylic acid (PAA), which is capable of analysis by TEM imaging.The PU formulation preferably includes a prepolymer, a chain extender, acatalyst and an adhesion promoter. Lightly pressing the mold againstthis layer causes the liquid PU prepolymer to flow and conform to therelief features on the mold. Passing light through the transparent moldcauses the PU to undergo chain extension and crosslinking to yield a setPU with Shore D hardness in the range of 60. Following curing of themolding material, the mold is removed to reveal a replica of thefeatures formed on the first substrate.

Direct AFM characterization of the surface of the PU reveals, withatomic scale precision, the vertical dimensions of the imprinted relief.As outlined above, the curing is preferably a photo-curing, althoughother curing techniques may be utilized by the process of the presentinvention.

FIG. 2 shows AFM images of a SWNT applied to a first substrate andcorresponding regions of three different PU structures imprinted with asingle mold derived from the first substrate or master. Qualitatively,the data shows that the process of the present invention accuratelyreproduces the nanoscale features associated with the SWNTs, even formultiple imprinting cycles. The Y-shaped SWNT junction, as well as thesmaller tube fragments on the first substrate, are all visible in eachPU sample. Line scans collected from the lower left branch of the “Y”structure (FIG. 2 insets) show that the imprinted relief features haveheights that are similar to those on the master. Some of the apparentdistortions in the cross sectional shapes of these features can beattributed to AFM artifacts associated with the roughness on the surfaceof the molded PU. This roughness has a root mean squared amplitude(evaluated by AFM) of 0.37 nm for Replica 1, and 0.4 nm for Replica 2and Replica 3. The maximum peak to valley height change associated withthis roughness is in the range of ˜1.5 nm.

Images obtained by AFM only reveal accurately the heights of the relieffeatures. Transmission electron microscopy (TEM) can determine theirwidths. Polyacrylic acid (PAA), rather than PU, was imprinted for thispurpose since PAA is a well-established polymer for TEM analysis. FIG. 3a shows an AFM image of a PAA layer imprinted with the same PDMS moldused for the results of FIG. 2, evaluated in the same region. Thereplication fidelity, heights of features, surface roughness and otherproperties are similar to those observed in PU. Depositing (at 30degrees to the surface of the PAA) Pt/C (to provide contrast in the TEM)and then C (at normal incidence, to provide structural support for thePt) on the imprinted PAA and then dissolving the PAA with watergenerates a Pt/C membrane replica of the relief structure. Forcomparison, a similar Pt/C replica was prepared from a SWNT master byetching away the SiO2 layer (with 2% HF in H2O) to lift off the replica.FIG. 3 shows TEM images of both types of replicas. The dark and brightstripes along the tube features represent regions of metal build-up andshadows, respectively. The separation between the darkest and brightestregions approximately defines the width of the feature. The separationwas measured by analyzing lines scans of the images averaged overstraight lengths (50 nm) of the relief features of interest. Theprofiles determined in this way from the imprinted PAA and SWNT mastersexhibit similar shapes. In both cases, there was observed a range ofwidths, between ˜3 nm to ˜10 nm. Widths below 3 nm were difficult todetermine due, at least in part, to the apparent grain size (˜1 nm) ofthe Pt/C. The TEM data of the replicas is consistent with the dimensionsand cross sectional shapes of the master.

From the AFM and TEM images presented in FIGS. 2-4, it is clear thatSWNTs with diameters larger than 2.5 nm on the master appear reliably ascontinuous replicated features in the replicas. The heights of thesefeatures vary, however, along their lengths from a maximum that isroughly equal to the height of the SWNTs on the master to a minimum thatis comparable to this height reduced by a value that is comparable tothe peak-to-valley surface roughness. The roughness plays a role in, andis indicative of, the polymer physics that limits the resolution.

The roughness, including line edge roughness and peak to valleyroughness contributes to the occurrence of breaks, or apparently missingsections, that begin to appear in AFM images of relief featuresassociated with SWNT diameters <2 nm, as seen in FIG. 4. In a preferredaspect of the present invention, the line edge roughness and surfaceroughness is less than 2 nanometers. For 1-2 nm diameter SWNTs, thesebreaks represent a substantial fraction of the overall length of theimprinted structures. Below 1 nm, only small fractions of the replicatedstructures are visible, as shown in FIG. 4 a. However, even at the ˜1 nmscale, it is still possible to identify the replicated relief byaveraging AFM line cuts collected along the length of a feature, asillustrated in FIG. 4 b. The resolution limits can be summarized byplotting the position averaged relief height as a function of SWNTdiameter, as illustrated in FIG. 4 a-e. The ultimate resolution iscorrelated to the ability of the prepolymer (PDMS, PU or PAA) to conformto the surface (master or PDMS mold) and the ability of the polymers(PDMS, PU or PAA) to retain the molded shape. Three pieces of datasuggest that the PDMS molds limit the resolution. First, breaks, shownin FIGS. 4 f-h, in the molded relief features typically occur at thesame positions in multiple molding cycles. Second, imprinted structuresin dissimilar polymers (i.e. PU and PAA) have similar surface roughnessand relief height distributions. Third, PU molded against a barefluorinated SiO2/Si wafer produces a surface roughness (0.19 nm) that issmaller than that generated with flat PDMS molds derived from these samewafers.

For extremely high resolution features there are at least two importantlength scales affecting the resolution limits of a replica: (i) theaverage distance between crosslinks, which is approximately ˜1 nm forthe h-PDMS, and (ii) the chemical bond lengths, which are in the rangeof 0.2 nm. A correlation of the above length scales and the resolutionlimits for features using different PDMS polymers is presented inTable 1. Although it is difficult to assign the observed roughness andresolution limits to particular molecular features of the PDMS, thedensity of crosslinks is likely to be a critical parameter. The averagemolecular weight between crosslinks (Mc) and distance between crosslinks(D) were determined by swelling samples in toluene and applyingFlory-Huggins theory. Table 1 summarizes the Mc and D values, andexperimental resolution limits and roughness parameters for h-PDMS, alow crosslink density version of this material (hl-PDMS) and acommercially available low modulus PDMS (s-PDMS). These three materialsexhibit a qualitative correlation between resolution and cross linkdensity. They also show that the resolution and roughness are related;both are influenced by the conformability of the polymer chains and theability of the crosslinked polymer to retain the molded shape. Attemptsto improve the resolution by increasing the number of crosslinks in theh-PDMS failed due to a tendency of the resulting material to stick tothe SWNT first substrates.

TABLE 1 Theoretical Peak to cross link rms valley Approximate distanceTheoretical Experimental roughness roughness Resolution (nm) Mc (g/mol)Mc (g/mol) (nm) (nm) limit h-PDMS 1.28 357 377 0.37 1.7 2 hl-PDMS 1.6554 536 0.54 3.1 3 s-PDMS 2.7 1239 891 0.54 3.2 3.5

EXAMPLES

The following example procedures were utilized to produce thenano-molded articles analyzed and shown in FIGS. 2-4.

Preparation of carbon nanotube master: A silicon wafer with a 100 nmthick layer of SiO2 (thermally grown) provided a substrate for SWNTgrowth. Ferritin catalyst (Aldrich) diluted by deionized water at avolumetric ratio of 1:1000 was cast onto the wafer. This wafer was thenimmediately placed into a quartz tube furnace at 800° C. for 2 minfollowed by purging with hydrogen gas at 900° C. for 1 min. Flowingmethane (500 standard cubic centimeters per minute (sccm)) and hydrogen(75 sccm) through the quartz tube at 900° C. for 10 min grows the SWNT.

Forming Mold: The SWNT/SiO2/Si master was placed in a vacuum chamberalong with 100 μL of(tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane (UnitedChemical Tech) for 2 hr. The resulting silane layer (monolayer orsub-monolayer coverage is expected) prevents adhesion of the PDMS to thebare SiO2. h-PDMS (Gelest, Inc) was prepared as following: 3.4 g (7-8%vinylmethylsiloxane) (Dimethylsiloxane), 100 μg of(1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane) and 50 μg ofplatinum catalyst were mixed and placed in vacuum chamber for 5 min. 1 g(25-30% methylhydrosiloxane) (Dimethylsiloxane) was then added, mixedand then the resulting sample was placed back into vacuum for 5 min.This prepolymer mixture was spin cast onto the SWNT master at 1000 rpmfor 40 s and then baked at 65° C. for 4 min. s-PDMS (Sylgard 184, DowCorning), prepared by mixing base and curing agent at a ratio of 10:1was then poured onto the h-PDMS. Baking at 65° C. for 2 hr completed thecuring of the polymers. Typical thicknesses were 10 μm for the h-PDMSand 3 mm for the s-PDMS.

Molding material: The polyurethane (PU) (NOA 73, Norland Products) wasspin cast onto a SiO2/Si wafer at 9000 rpm for 40 s. The mold was placedonto this thin film and pressed gently to ensure good wetting at theinterface. Exposing the PU to ultraviolet light (350-380 nm; long waveultraviolet lamp, UVP) at about 19 mw/cm2 for 1 hr through the moldcured the PU and solidified the film.

Measurement analysis: Characterization of the SWNT master and imprintedPU structure was carried out by AFM (Dimension 3100, Digital Instrument)and TEM (Philips CM200, FEI). The TEM pictures of the metal shadowedreplicas were taken at 120 kV. The AFM measurements were executed intapping mode with tips (BS-Tap300Al) from BudgetSensors. The resonantfrequency of the tip was 300 kHz.

Preparation of TEM sample: Several drops of a methanol solution ofpolyacrylic acid (PAA) (30% wt) were placed onto a PDMS mold. The samplewas left in this configuration, in open air, until the methanolevaporated (˜10 hours was typically sufficient). The PAA film was thenpeeled away leaving an imprinted nanostructure on the surface of thePAA. The sample was placed in the vacuum chamber of a thermalevaporator. The Pt/C source was located above the sample at an elevationangle of 30 degree. A few nanometers of Pt/C were deposited on thesample. Subsequently, carbon film with thickness of ˜10 nm wasevaporated at normal incidence on the sample. The sample was soaked inDI water for several hours until PAA was dissolved. The Pt/C and carbonfilm was then floated on the water surface. They were later collected bya TEM copper mesh.

The invention has been described in an illustrative manner, and it is tobe understood that the terminology that has been used is intended to bein a nature of description rather than limitation.

Many modifications and variations of the present invention are possiblein light of the above teachings. It is therefore, to be understood thatwithin the scope of the appended claims, the invention may be practicedother than as specifically described.

1. A nano-molding process comprising: forming a first substrate havingnano-scale features formed thereon, wherein the nano-scale features havelateral and vertical dimensions of less than ten nanometers; forming amold against the first substrate; removing the mold from the firstsubstrate; providing a second substrate having a molding materialapplied thereon; pressing the mold against the second substrate allowingthe molding material to conform to a shape of the mold; curing themolding material; and removing the mold from the second substrate havingthe cured molding material revealing a replica of the first substrate.2. The nano-molding process of claim 1, wherein forming the mold furthercomprises: casting a first polymer against the first substrate;partially curing the first polymer; applying a second polymer to thefirst polymer; and curing the first and second polymer to form the mold.3. The nano-molding process of claim 2 wherein the first polymercomprises h-polydimethylsiloxane.
 4. The nano-molding process of claim 2wherein the second polymer comprises s-polydimethylsiloxane.
 5. Thenano-molding process of claim 1 wherein the first substrate hasnano-scale features having dimensions greater than 2 nanometers.
 6. Thenano-molding process of claim 5 wherein the dimensions are from 2 to 7nanometers.
 7. The nano-molding process of claim 1 wherein the step offorming the first substrate includes forming single walled carbonnanotubes on a silicon wafer.
 8. The nano-molding process of claim 1wherein the molding material comprises a photo curable material.
 9. Thenano-molding process of claim 8 wherein the photocurable material isselected from the group consisting of: polyurethanes or vinyl-functionalmonomers.
 10. The nano-molding process of claim 1 further comprising:verifying the dimensions of the replica.
 11. The nano-molding process ofclaim 10 wherein verifying comprises: measuring a vertical dimension onthe first substrate; measuring a vertical dimension on the replica; andcomparing the vertical measurements of the first substrate and replica.12. The nano-molding process of claim 11 wherein the vertical dimensionis measured using atomic force microscopy.
 13. The nano-molding processof claim 10 wherein verifying comprises: measuring a lateral dimensionon the first substrate; measuring a lateral dimension on the replica;and comparing the lateral measurements of the first substrate andreplica.
 14. The nano-molding process of claim 13 wherein the lateraldimension is measured using transmission electron microscopy.
 15. Thenano-molding process of claim 1, wherein the replica has a line-edgeroughness less than 2 nanometers.
 16. The nano-molding process of claim1, wherein the nano-scale features of the first substrate have a minimallateral dimension of less than 7 nanometers.
 17. The nano-moldingprocess of claim 1, wherein an inherent surface roughness as measured bythe peak-to-valley surface roughness of the material is less than 2nanometers.